Generation 0 – Objects that have been created since the last GC pass (newest objects)

Generation 1 – Objects that have survived one pass of the GC

Generation 2 – All other objects (oldest objects)

Objects are only promoted to the next generation if the generation that they are currently located in is examined and collected during a garbage collection pass.

This means:

Since the GC always examines Gen 0 during any GC pass, Gen 0 objects that survive a garbage collection are always promoted to Gen 1.

Objects in Gen 1 are only promoted to Gen 2 if Gen 1 is examined and collected during a GC pass and the object survives. The GC will very often do only a Gen 0 pass during collection. When the GC only examines and collects Gen 0, Gen 1 objects are not examined and therefore not promoted to Gen 2.